Re: [PATCH v3 5/6] ext4: introduce direct IO write path using iomap infrastructure

Ritesh Harjani
2019-09-16T04:37:55Z

Hello,

On 9/12/19 4:34 PM, Matthew Bobrowski wrote:
> This patch introduces a new direct IO write code path implementation
> that makes use of the iomap infrastructure.
>
> All direct IO write operations are now passed from the ->write_iter()
> callback to the new function ext4_dio_write_iter(). This function is
> responsible for calling into iomap infrastructure via
> iomap_dio_rw(). Snippets of the direct IO code from within
> ext4_file_write_iter(), such as checking whether the IO request is
> unaligned asynchronous IO, or whether it will ber overwriting
> allocated and initialized blocks has been moved out and into
> ext4_dio_write_iter().
>
> The block mapping flags that are passed to ext4_map_blocks() from
> within ext4_dio_get_block() and friends have effectively been taken
> out and introduced within the ext4_iomap_begin(). If ext4_map_blocks()
> happens to have instantiated blocks beyond the i_size, then we attempt
> to place the inode onto the orphan list. Despite being able to perform
> i_size extension checking earlier on in the direct IO code path, it
> makes most sense to perform this bit post successful block allocation.
>
> The ->end_io() callback ext4_dio_write_end_io() is responsible for
> removing the inode from the orphan list and determining if we should
> truncate a failed write in the case of an error. We also convert a
> range of unwritten extents to written if IOMAP_DIO_UNWRITTEN is set
> and perform the necessary i_size/i_disksize extension if the
> iocb->ki_pos + dio->size > i_size_read(inode).
>
> In the instance of a short write, we fallback to buffered IO and
> complete whatever is left the 'iter'. Any blocks that may have been
> allocated in preparation for direct IO will be reused by buffered IO,
> so there's no issue with leaving allocated blocks beyond EOF.
>
> Signed-off-by: Matthew Bobrowski <mbobrowski@mbobrowski.org>
> Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
> ---
> fs/ext4/file.c | 219 +++++++++++++++++++++++++++++++++---------------
> fs/ext4/inode.c | 57 ++++++++++---
> 2 files changed, 196 insertions(+), 80 deletions(-)
>
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index 6457c629b8cf..413c7895aa9e 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -29,6 +29,7 @@
> #include <linux/pagevec.h>
> #include <linux/uio.h>
> #include <linux/mman.h>
> +#include <linux/backing-dev.h>
> #include "ext4.h"
> #include "ext4_jbd2.h"
> #include "xattr.h"
> @@ -213,12 +214,16 @@ static ssize_t ext4_write_checks(struct kiocb *iocb, struct iov_iter *from)
> struct inode *inode = file_inode(iocb->ki_filp);
> ssize_t ret;
>
> + if (unlikely(IS_IMMUTABLE(inode)))
> + return -EPERM;
> +
> ret = generic_write_checks(iocb, from);
> if (ret <= 0)
> return ret;
>
> - if (unlikely(IS_IMMUTABLE(inode)))
> - return -EPERM;
> + ret = file_modified(iocb->ki_filp);
> + if (ret)
> + return 0;

Why not return ret directly, otherwise we will be returning the wrong
error code to user space. Thoughts?

Do you think simplification/restructuring of this API
"ext4_write_checks" can be a separate patch, so that this patch
only focuses on conversion of DIO write path to iomap?

Also, I think we can make the function (ext4_write_checks())
like below.
This way we call for file_modified() only after we have checked for
write limits, at one place.

static ssize_t ext4_write_checks(struct kiocb *iocb, struct iov_iter
*from)
{
struct inode *inode = file_inode(iocb->ki_filp);
ssize_t ret;

if (unlikely(IS_IMMUTABLE(inode)))
return -EPERM;

ret = generic_write_checks(iocb, from);
if (ret <= 0)
_ return ret;
/*
* If we have encountered a bitmap-format file, the size limit
* is smaller than s_maxbytes, which is for extent-mapped files.
*/
if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))) {
struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);

if (iocb->ki_pos >= sbi->s_bitmap_maxbytes)
return -EFBIG;
iov_iter_truncate(from, sbi->s_bitmap_maxbytes -
iocb->ki_pos);
}
+
+ ret = file_modified(iocb->ki_filp);
+ if (ret)
+ return ret;
+
return iov_iter_count(from);
}

-ritesh

>
> /*
> * If we have encountered a bitmap-format file, the size limit
> @@ -234,6 +239,32 @@ static ssize_t ext4_write_checks(struct kiocb *iocb, struct iov_iter *from)
> return iov_iter_count(from);
> }
>
> +static ssize_t ext4_buffered_write_iter(struct kiocb *iocb,
> + struct iov_iter *from)
> +{
> + ssize_t ret;
> + struct inode *inode = file_inode(iocb->ki_filp);
> +
> + if (iocb->ki_flags & IOCB_NOWAIT)
> + return -EOPNOTSUPP;
> +
> + inode_lock(inode);
> + ret = ext4_write_checks(iocb, from);
> + if (ret <= 0)
> + goto out;
> +
> + current->backing_dev_info = inode_to_bdi(inode);
> + ret = generic_perform_write(iocb->ki_filp, from, iocb->ki_pos);
> + current->backing_dev_info = NULL;
> +out:
> + inode_unlock(inode);
> + if (likely(ret > 0)) {
> + iocb->ki_pos += ret;
> + ret = generic_write_sync(iocb, ret);
> + }
> + return ret;
> +}
> +
> static int ext4_handle_inode_extension(struct inode *inode, loff_t offset,
> ssize_t len, size_t count)
> {
> @@ -310,6 +341,120 @@ static int ext4_handle_failed_inode_extension(struct inode *inode, loff_t size)
> return 0;
> }
>
> +/*
> + * For a write that extends the inode size, ext4_dio_write_iter() will
> + * wait for the write to complete. Consequently, operations performed
> + * within this function are still covered by the inode_lock(). On
> + * success, this function returns 0.
> + */
> +static int ext4_dio_write_end_io(struct kiocb *iocb, ssize_t size, int error,
> + unsigned int flags)
> +{
> + int ret;
> + loff_t offset = iocb->ki_pos;
> + struct inode *inode = file_inode(iocb->ki_filp);
> +
> + if (error) {
> + ret = ext4_handle_failed_inode_extension(inode, offset + size);
> + return ret ? ret : error;
> + }
> +
> + if (flags & IOMAP_DIO_UNWRITTEN) {
> + ret = ext4_convert_unwritten_extents(NULL, inode,
> + offset, size);
> + if (ret)
> + return ret;
> + }
> +
> + if (offset + size > i_size_read(inode)) {
> + ret = ext4_handle_inode_extension(inode, offset, size, 0);
> + if (ret)
> + return ret;
> + }
> + return 0;
> +}
> +
> +static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
> +{
> + ssize_t ret;
> + size_t count;
> + loff_t offset = iocb->ki_pos;
> + struct inode *inode = file_inode(iocb->ki_filp);
> + bool extend = false, overwrite = false, unaligned_aio = false;
> +
> + if (!inode_trylock(inode)) {
> + if (iocb->ki_flags & IOCB_NOWAIT)
> + return -EAGAIN;
> + inode_lock(inode);
> + }
> +
> + if (!ext4_dio_checks(inode)) {
> + inode_unlock(inode);
> + /*
> + * Fallback to buffered IO if the operation on the
> + * inode is not supported by direct IO.
> + */
> + return ext4_buffered_write_iter(iocb, from);
> + }
> +
> + ret = ext4_write_checks(iocb, from);
> + if (ret <= 0) {
> + inode_unlock(inode);
> + return ret;
> + }
> +
> + /*
> + * Unaligned direct AIO must be serialized among each other as
> + * the zeroing of partial blocks of two competing unaligned
> + * AIOs can result in data corruption.
> + */
> + if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS) &&
> + !is_sync_kiocb(iocb) && ext4_unaligned_aio(inode, from, offset)) {
> + unaligned_aio = true;
> + inode_dio_wait(inode);
> + }
> +
> + /*
> + * Determine whether the IO operation will overwrite allocated
> + * and initialized blocks. If so, check to see whether it is
> + * possible to take the dioread_nolock path.
> + */
> + count = iov_iter_count(from);
> + if (!unaligned_aio && ext4_overwrite_io(inode, offset, count) &&
> + ext4_should_dioread_nolock(inode)) {
> + overwrite = true;
> + downgrade_write(&inode->i_rwsem);
> + }
> +
> + if (offset + count > i_size_read(inode) ||
> + offset + count > EXT4_I(inode)->i_disksize) {
> + ext4_update_i_disksize(inode, inode->i_size);
> + extend = true;
> + }
> +
> + ret = iomap_dio_rw(iocb, from, &ext4_iomap_ops, ext4_dio_write_end_io);
> +
> + /*
> + * Unaligned direct AIO must be the only IO in flight or else
> + * any overlapping aligned IO after unaligned IO might result
> + * in data corruption. We also need to wait here in the case
> + * where the inode is being extended so that inode extension
> + * routines in ext4_dio_write_end_io() are covered by the
> + * inode_lock().
> + */
> + if (ret == -EIOCBQUEUED && (unaligned_aio || extend))
> + inode_dio_wait(inode);
> +
> + if (overwrite)
> + inode_unlock_shared(inode);
> + else
> + inode_unlock(inode);
> +
> + if (ret >= 0 && iov_iter_count(from))
> + return ext4_buffered_write_iter(iocb, from);
> + return ret;
> +}
> +
> #ifdef CONFIG_FS_DAX
> static ssize_t
> ext4_dax_write_iter(struct kiocb *iocb, struct iov_iter *from)
> @@ -324,15 +469,10 @@ ext4_dax_write_iter(struct kiocb *iocb, struct iov_iter *from)
> return -EAGAIN;
> inode_lock(inode);
> }
> +
> ret = ext4_write_checks(iocb, from);
> if (ret <= 0)
> goto out;
> - ret = file_remove_privs(iocb->ki_filp);
> - if (ret)
> - goto out;
> - ret = file_update_time(iocb->ki_filp);
> - if (ret)
> - goto out;
>
> offset = iocb->ki_pos;
> ret = dax_iomap_rw(iocb, from, &ext4_iomap_ops);
> @@ -358,73 +498,16 @@ static ssize_t
> ext4_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
> {
> struct inode *inode = file_inode(iocb->ki_filp);
> - int o_direct = iocb->ki_flags & IOCB_DIRECT;
> - int unaligned_aio = 0;
> - int overwrite = 0;
> - ssize_t ret;
>
> if (unlikely(ext4_forced_shutdown(EXT4_SB(inode->i_sb))))
> return -EIO;
>
> -#ifdef CONFIG_FS_DAX
> if (IS_DAX(inode))
> return ext4_dax_write_iter(iocb, from);
> -#endif
> - if (!o_direct && (iocb->ki_flags & IOCB_NOWAIT))
> - return -EOPNOTSUPP;
>
> - if (!inode_trylock(inode)) {
> - if (iocb->ki_flags & IOCB_NOWAIT)
> - return -EAGAIN;
> - inode_lock(inode);
> - }
> -
> - ret = ext4_write_checks(iocb, from);
> - if (ret <= 0)
> - goto out;
> -
> - /*
> - * Unaligned direct AIO must be serialized among each other as zeroing
> - * of partial blocks of two competing unaligned AIOs can result in data
> - * corruption.
> - */
> - if (o_direct && ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS) &&
> - !is_sync_kiocb(iocb) &&
> - ext4_unaligned_aio(inode, from, iocb->ki_pos)) {
> - unaligned_aio = 1;
> - ext4_unwritten_wait(inode);
> - }
> -
> - iocb->private = &overwrite;
> - /* Check whether we do a DIO overwrite or not */
> - if (o_direct && !unaligned_aio) {
> - if (ext4_overwrite_io(inode, iocb->ki_pos, iov_iter_count(from))) {
> - if (ext4_should_dioread_nolock(inode))
> - overwrite = 1;
> - } else if (iocb->ki_flags & IOCB_NOWAIT) {
> - ret = -EAGAIN;
> - goto out;
> - }
> - }
> -
> - ret = __generic_file_write_iter(iocb, from);
> - /*
> - * Unaligned direct AIO must be the only IO in flight. Otherwise
> - * overlapping aligned IO after unaligned might result in data
> - * corruption.
> - */
> - if (ret == -EIOCBQUEUED && unaligned_aio)
> - ext4_unwritten_wait(inode);
> - inode_unlock(inode);
> -
> - if (ret > 0)
> - ret = generic_write_sync(iocb, ret);
> -
> - return ret;
> -
> -out:
> - inode_unlock(inode);
> - return ret;
> + if (iocb->ki_flags & IOCB_DIRECT)
> + return ext4_dio_write_iter(iocb, from);
> + return ext4_buffered_write_iter(iocb, from);
> }
>
> #ifdef CONFIG_FS_DAX
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index efb184928e51..f52ad3065236 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3513,11 +3513,13 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
> }
> }
> } else if (flags & IOMAP_WRITE) {
> - int dio_credits;
> handle_t *handle;
> - int retries = 0;
> + int dio_credits, retries = 0, m_flags = 0;
>
> - /* Trim mapping request to maximum we can map at once for DIO */
> + /*
> + * Trim mapping request to maximum we can map at once
> + * for DIO.
> + */
> if (map.m_len > DIO_MAX_BLOCKS)
> map.m_len = DIO_MAX_BLOCKS;
> dio_credits = ext4_chunk_trans_blocks(inode, map.m_len);
> @@ -3533,8 +3535,30 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
> if (IS_ERR(handle))
> return PTR_ERR(handle);
>
> - ret = ext4_map_blocks(handle, inode, &map,
> - EXT4_GET_BLOCKS_CREATE_ZERO);
> + /*
> + * DAX and direct IO are the only two operations that
> + * are currently supported with IOMAP_WRITE.
> + */
> + WARN_ON(!IS_DAX(inode) && !(flags & IOMAP_DIRECT));
> + if (IS_DAX(inode))
> + m_flags = EXT4_GET_BLOCKS_CREATE_ZERO;
> + else if (round_down(offset, i_blocksize(inode)) >=
> + i_size_read(inode))
> + m_flags = EXT4_GET_BLOCKS_CREATE;
> + else if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))
> + m_flags = EXT4_GET_BLOCKS_IO_CREATE_EXT;
> +
> + ret = ext4_map_blocks(handle, inode, &map, m_flags);
> +
> + /*
> + * We cannot fill holes in indirect tree based inodes
> + * as that could expose stale data in the case of a
> + * crash. Use the magic error code to fallback to
> + * buffered IO.
> + */
> + if (!m_flags && !ret)
> + ret = -ENOTBLK;
> +
> if (ret < 0) {
> ext4_journal_stop(handle);
> if (ret == -ENOSPC &&
> @@ -3544,13 +3568,14 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
> }
>
> /*
> - * If we added blocks beyond i_size, we need to make sure they
> - * will get truncated if we crash before updating i_size in
> - * ext4_iomap_end(). For faults we don't need to do that (and
> - * even cannot because for orphan list operations inode_lock is
> - * required) - if we happen to instantiate block beyond i_size,
> - * it is because we race with truncate which has already added
> - * the inode to the orphan list.
> + * If we added blocks beyond i_size, we need to make
> + * sure they will get truncated if we crash before
> + * updating the i_size. For faults we don't need to do
> + * that (and even cannot because for orphan list
> + * operations inode_lock is required) - if we happen
> + * to instantiate block beyond i_size, it is because
> + * we race with truncate which has already added the
> + * inode to the orphan list.
> */
> if (!(flags & IOMAP_FAULT) && first_block + map.m_len >
> (i_size_read(inode) + (1 << blkbits) - 1) >> blkbits) {
> @@ -3612,6 +3637,14 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
> static int ext4_iomap_end(struct inode *inode, loff_t offset, loff_t length,
> ssize_t written, unsigned flags, struct iomap *iomap)
> {
> + /*
> + * Check to see whether an error occurred while writing data
> + * out to allocated blocks. If so, return the magic error code
> + * so that we fallback to buffered IO and reuse the blocks
> + * that were allocated in preparation for the direct IO write.
> + */
> + if (flags & IOMAP_DIRECT && written == 0)
> + return -ENOTBLK;
> return 0;
> }
>